The discovery of giant trilobites in northern Portugal reveals the once ubiquitous marine creature mated en masse and used its numbers for protection, say European researchers.

The new find, published in the current issue of the journal Geology describes giants that grew to 90 centimetres in length, the largest ever found.

Trilobites once roamed the sea floor, but were wiped out in the Permian-Triassic extinction, 250 million years ago.

These marine arthropods, typically less than 8 centimetres long, are distant relatives of modern-day lobsters and spiders.

The researchers, led by Dr Juan Carlos Gutiérrez-Marco from the El Instituto de Geología Económica in Madrid, Spain, discovered trilobites from 15 genera in 465-million year old rocks in Arouca Geopark in northern Portugal.

They found a complete specimen 70 centimetres in length and others whose tail remnants indicated they grew to up to 90 centimetres long.

Most of the trilobite species they collected have been found elsewhere in Western Europe, but never before of such giant size.

Their size was probably an adaption to the polar waters where they dwelt, say the researchers.

"Metabolism of invertebrates is slower in cold water, so it takes longer to reach adulthood and they also tend to live longer. Also if you are bigger you are better able to deter a predator attack," sasy co-author Dr Diego García-Bellido, also of the El Instituto de Geología Económica.

Molt together, mate together

The researchers found clusters of trilobites with up to 1000 individuals, indicating they grouped together to molt, much like modern-day horseshoe crabs.

The researchers assume that like horseshoe crabs, the trilobites may have also mated en masse. The hormones that instigate molting are related to those that induce sexual reproduction, says García-Bellido.

Several trilobites in the deposit were also found in burrows and under the shells of larger organisms, where they may have hidden after molting as their soft bodies made them more vulnerable to predators.

The trilobites are believed to have died when the stagnant seawater became oxygen-depleted, which also helped their preservation as fossils, the researchers say.

Spectacular find

"It's really exceptional in that you rarely find trilobite fossils complete," he says. "Mostly you find a piece of the head or the tail, so to find them in congregations where there are many complete individuals is astounding."

Paterson says his own research in the Flinders Ranges in South Australia has shown that trilobites grouped together during the Cambrian era about 520 million years ago, when the diversity of life really kicked off.

"You've got a better chance of survival if you are [molting] in a group as you've got less chance of being picked off yourself.